Portable electronic devices such as a camcorder, a mobile phone and a laptop computer have been rapidly improved in compactness and weight reduction. As a driving power source for these devices, non-aqueous electrolyte secondary cells, which have high energy density and high capacity, are widely used.
Conventionally, as a positive electrode active material for a non-aqueous electrolyte secondary cell, lithium cobalt composite oxide (LiCoO2) has been used because of its excellent discharge characteristics. However, cobalt is expensive because its abundance is small. Therefore, there has been focused on technologies of an active material using nickel (LiaNibCocMndO2, 0.9≦a≦1.2, 0<b, b+c+d=1) because nickel is more abundant and cheaper than cobalt.
For the purpose of preparing a nickel-containing active material, it is needed that a source of transition metals (nickel source, cobalt source, etc.) and a lithium source having an amount more than required are mixed, and that this mixture is calcined at a lower temperature than that for preparation of lithium cobalt composite oxide.
However, in such a method, an unreacted lithium source and a lithium compound such as a calcined lithium source are likely to remain on the surface of the calcined active material. These lithium compounds are reacted with the non-aqueous electrolyte to produce byproducts that adversely affect on charge/discharge reaction. Such a reaction is likely to occur particularly in case of high rate discharge or storage under a high temperature environment, leading to deterioration in high temperature storage characteristics and load discharge characteristics.
For reference, techniques of non-aqueous electrolyte secondary cells are disclosed in the following Patent Documents 1 to 8.
[Patent Documents]
[Patent Document 1]
    Japanese Patent Application Publication No. 2008-243448[Patent Document 2]    Japanese Patent Application Publication No. 2010-73686[Patent Document 3]    Japanese Patent Application Publication No. 2007-42302[Patent Document 4]    International Publication WO 2007/102407[Patent Document 5]    Japanese Patent No. 4,082,214[Patent Document 6]    Japanese Patent Application Publication No. 2006-120650[Patent Document 7]    Japanese Patent Application Publication No. 2009-176528[Patent Document 8]    Japanese Patent Application Publication No. 2008-277086
Patent Document 1 discloses a technique using a lithium transition metal composite oxide represented by the general formula (1) shown below. In a pore distribution curve of secondary particles of the lithium transition metal composite oxide measured by a mercury press-fitting method, there are a main peak top at a pore radius of more than 1 μm and 50 μm or less, and a sub peak top at a pore radius of 0.08 μm or more and 1 μm or less.LiXNiαMnβCoYQδWYO2  (1)(Wherein, Q represents at least one element selected from Al, Fe, Ga, Sn, V, Cr, Cu, Zn, Mg, Ti, Ge, B, Bi, Nb, Ta, Mo, Zr, Ca and Mo. And the following formulas are satisfied: 0.2≦α≦0.6, 0.2≦β≦0.6, 0≦Y≦0.5, 0≦δ≦0.1, 0.8≦α+β+Y+δ≦1.2, 0<x≦1.2, 0<Y≦0.1.)
The document states that this technique provides, at low cost, a lithium transition metal composite oxide that has high performances (high capacity, high rate characteristics, resistance characteristics, etc.) and is suitable for a positive electrode material of lithium secondary cells.
Patent Document 2 discloses a technique using, as an active material, particles of a lithium-containing composite oxide represented by the general formula: Li1+xNi(1−y−z+b)/2Mn(1−y−z−b)/2CoyMzO2 (wherein M represents at least one element selected from the group consisting of Ti, Cr, Fe, Cu, Zn, Al, Ge, Sn, Mg, Ag, Ta, Nb, B, P, Zr, W and Ga; −0.15≦x≦0.15, 0≦y≦0.4, 0≦z≦0.03, −0.1≦b≦0.96, and 1−y−z−b>0). In this composite oxide, Ni has an average valence of 2.2 to 2.9, and primary particles having a particle size of 1 μm or less are contained at 30% by volume or less relative to the total primary particles, and BET surface area is 0.3 m2/g or less.
The document states that this technique realizes a non-aqueous secondary cell having high capacity and excellent thermal stability.
Patent Document 3 discloses a technique using a positive electrode containing a first positive electrode material having an average composition represented by the chemical formula 1 and a second positive electrode material having an average composition represented by the chemical formula 2.LiaCo1-bM1bO2-c  (Chemical formula 1)(wherein M1 represents at least one of the group consisting of manganese (Mn), nickel (Ni), magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), gallium (Ga), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr) and tungsten (W). The values a, b and c are within the following ranges: 0.9≦a≦1.1, 0≦b≦0.3, −0.1≦c≦0.1.)LiwNixCoyMnzM21−x−y−zO2−v  (Chemical formula 2)(wherein M2 represents at least one of the group consisting of magnesium (Mg), aluminum (Al), boron (B), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), copper (Cu), zinc (Zn), gallium (Ga), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), tin (Sn), calcium (Ca), strontium (Sr) and tungsten (W). The values v, w, x, y and z are within the following ranges: 31 0.1≦v≦0.1, 0.9<w≦1.1, 0<x<1, 0<y<0.7, 0<z<0.5, 0≦1−x−y−z≦0.2.)
The document states that this technology can improve charge/discharge efficiency in addition to energy density.
Patent Document 4 discloses a technique using, as a positive electrode active material, a lithium-containing composite oxide powder represented by the general formula LipNxMyOzFa (wherein N is at least one element selected from the group consisting of Co, Mn and Ni. M is at least one element selected from the group consisting of Al, alkaline earth metal elements, and transition metal elements other than the element N. 0.9≦p≦1.1, 0.965≦x<1.00, 0<y≦0.035, 1.9≦z≦2.1, x+y=1, 0≦a≦0.02). The lithium-containing composite oxide powder contains zirconium in its surface layer, and an atomic ratio (zirconium/the element N) within 5 nm from the surface layer 1.0 or more.
The document states that this technique can realize a non-aqueous electrolyte secondary cell having high operating voltage, high volume capacity density, high safety, and excellent charge/discharge cycle characteristics.
Patent Document 5 discloses a technique using a lithium composite oxide represented by the composition formula below (a) as a positive electrode active material. This lithium composite oxide shows X-ray diffraction pattern including a diffraction peak of a composite oxide of Li and W and/or of Li and Mo in addition to a main diffraction peak attributed to a hexagonal crystal structure.LiaNibCocMndMeO2  (a)(wherein M refers to one or two of W and Mo, 0.90≦a≦1.15, 0<b<0.99, 0<c≦0.5, 0<d≦0.5, 0<c+d≦0.9, 0.01≦e≦0.1, and b+c+d+e=1 (however, when b+c+d=x, the following ranges are excluded: 1.00x≦a≦1.15x, 0.45x<b<0.94x, 0.05x<c≦0.35x, 0.01x≦d≦0.2x, 0.06x≦c+d≦0.55x, and 0.0001x≦e≦0.03x.).
The document states that this technique can achieve a high performance cell having high initial capacity and keeping good thermal stability even after charging.
Patent Document 6 discloses a technique in which at least one of cyclohexylbenzene and tert-alkylbenzene derivatives is contained in a non-aqueous electrolyte solution in a total concentration of 0.1 to 10% by mass relative to the electrolyte solution.
The document states that this technique can achieve a lithium secondary cell that excels in cell characteristics including cycle characteristics, electric capacity, storage characteristics, and safety such as anti-overcharge, etc.
Patent Document 7 discloses a technique in which a lithium nickel composite oxide is used as a positive electrode active material, and lithium fluoroborate (LiBF4) and tert-amylbenzene (TAB) are added to a non-aqueous electrolyte. The lithium nickel composite oxide is prepared by water washing a lithium nickel composite oxide having a constant specific surface area (S). The specific surface area after the washing (S′) is 0.5 to 3.0 m2/g, and the ratio of the specific surface areas before and after the washing (S′/S) is 1.5 to 4.0.
The document states that this technique provides a cell having excellent charge/discharge cycle characteristics and high temperature storage characteristics.
Patent Document 8 disclose a technique in which a lithium cobalt composite oxide having at least one of magnesium (Mg) and zirconium (Zr) is contained in a positive electrode active material, and 0.5 to 3.0% by mass of 1,3-dioxane is contained in a nonaqueous electrolyte.
The document states that this technique can realize a non-aqueous electrolyte secondary cell having excellent temperature storage characteristics and high safety at overcharging.
However, even with these technologies, a problem exists that high temperature storage characteristics and load characteristics of non-aqueous electrolyte secondary cells using a nickel-containing active material cannot be sufficiently improved.